You won't be able to post the service using any of the standard operations on it. That is true for all of the services I'm aware of that create transactions. The operations that ship with out-of-the-box document services are create, read, update, delete, find and more types of reads. They don't have functions built in them to post. To get that functionality you'd need to either create a new service that isn't a document service and is rather a custom service or you'd have to modify the existing service and add a new operation to it.

If you are working in 2012, custom services are pretty easy to create so I wouldn't shy away from them. You pretty much just create an X++ class that contain methods to do whatever functionality you need (in this case posting) and add attributes to the class and method so they can be exposed for external consumption in a service. Have a look here for more details: